1. Field of the Invention
The present invention relates to a method for producing a low-density flexible polyurethane foam favorably used as a cushion material for bedclothes and furniture, a crating material and the like.
2. Description of the Prior Art
A low-density flexible polyurethane foam has conventionally been produced by mixing starting components such as polyhydroxy compounds, polyisocyanates, foaming agents and catalysts and then stirring the mixture to foam and resinify it. This production method utilizes a polycondensation reaction which is one of polymeric synthetic reactions and is an exothermic reaction. Particularly, when water is used as a foaming agent, the heat of the foaming reaction between a polyisocyanate and water greatly contributes to heat generation during production of the flexible polyurethane foam. In this case, the reaction heat is accumulated within the flexible polyurethane foam. Although the accumulated heat at the periphery portions of the polyurethane foam is easily radiated outward with the passing of time, the heat accumulation at the central portion is only influenced to a small extent by the peripheral heat radiation, as the size of polyurethane foam block becomes larger and the central portion reaches a maximum temperature (the maximum temperature of the internal heat generation of flexible polyurethane foam is hereafter referred to as Tmax), after 15 to 30 minutes from the end of the foaming reaction. The Tmax increases in proportion to the amount of water used as a foaming agent and, for example, using 5 parts by weight of water per 100 parts by weight of a polyhydroxy compound, results in a Tmax of 165.degree. to 170.degree. C. Radiation of the internal heat of flexible polyurethane foam is retarded, as its size increases and therefore, thermal deterioration and oxidational deterioration due to high temperature occur at the central portion of flexible polyurethane foam block maintained at high temperature for a long period of time, resulting in not only in foam discoloration called internal scorching and reduced physical properties, but also in spontaneous ignition in an extreme case.
A method for preventing the internal scorching of polyurethane foam and its spontaneous ignition due to high temperature by adding an antioxidant to a starting mixture has conventionally been proposed (Japanese Patent Laid-open No. 68,898/1979). However, this method cannot completely solve the above problems, since its objective is not to directly control the internal temperature of polyurethane foam, especially the high internal temperature resulting from the use of at least 5 parts by weight of water, but only to prevent the oxidational deterioration and the thermal deterioration of polyurethane foam at high temperature by using an antioxidant.
On the other hand, there is a known method for producing a flexible polyurethane foam at a lower Tmax by controlling its internal temperature, in which a reduced amount of water is used and a volatile organic solvent such monochlorotrifluoromethane or methylene chloride is added as an auxiliary foaming agent. This volatile organic solvent, which is incorporated in a liquid state into a starting mixture, evaporates during foaming thereby making a flexible polyurethane foam have lower density. This method has an effect of reducing the internal temperature of the foam by vaporization heat. However, since the use of monochlorotrifluoromethane tends to be restricted because of its problem of breaking the ozone layer and the use of organic solvents such as methylene chloride tends to be restricted because of their toxicity in recent years, a foaming method without any one of these volatile organic solvents is desired.
A method for producing a low-density flexible polyurethane foam is also proposed in which formic acid is used, other than water and volatile organic solvents, as a foaming agent (Japanese Patent Laid-open No. 29,837/1983). However, since heat generation also occurs during foaming when formic acid is used, the use of a large amount of formic acid results in the internal scorching of polyurethane foam and its spontaneous ignition, as in the case, when a large amount of water is used as a foaming agent.
There is another known method for producing a flexible polyurethane foam which comprises introducing polyhydroxy compounds and polyisocyanates into a mixing tank prior to mixing them while cooling them, then placing the mixture in a mixer and adding catalysts, foaming agents and the like to the mixture to foam it (Japanese Patent Laid-open No. 213,326/1989). In this method, however, the starting materials are not previously cooled and therefore a cooling mixture chamber is required in addition to conventional installations. Besides, the purpose of cooling is to improve the physical properties by retarding the reaction.